The proposed research will develop and apply the use of Ecological Momentary Assessment (EMA) to study situation-specific sequences of adolescents'peer affiliations, perceptions of peer acceptance, mood states, current activities, location, and adult monitoring. Existing evidence has shown that peers are the predominant influence on the development of problem behavior and substance use in early adolescence, particularly when youth are unsupervised and involved in unstructured activities. Applying EMA methodology to changing peer contexts and intrapersonal processes will help establish better understanding of the process of peer influence. The sequential nature of the EMA data can reveal antecedent-consequent relationships among behavioral and cognitive factors, such as the extent to which mood state and self perception of acceptance affects the strength of deviant peer influence. EMA data will be collected in real time with PDA technology by adolescents during non-school hours in natural settings. Such data are free from recall biases that are characteristic of typically used retrospective questionnaires. A sample of 96 youth will be selected from 4 control schools participating in an already funded research project that is evaluating Positive Behavior Support (PBS) in middle schools. Half of the participating youth will be at-risk for developing substance use and other behavior problems and half will be a universal sample;there will be equal numbers of boys and girls in the sample. One week of EMA data will be collected three times when participating youth are in seventh grade and once the following year. Youth self-report surveys of behavior and substance use, peer harassment and exclusion, and deviant peer affiliation will be available from the collaborating PBS project, as will school discipline referral data. Such data will have been collected the year prior to this proposed study (when students were in sixth grade), and will also be collected when participating youth are in seventh and eighth grades. In addition, bimonthly web-based surveys of peer networks, behavior, and peer harassment will be collected. This study will investigate the concurrent validity of the EMA measures of antisocial behavior, substance use, peer harassment and exclusion, and behavior of peer affiliates with the bimonthly web-based reports. This study will also investigate the predictive validity of these EMA measures on youth self-reports and school records of behavioral and substance use outcomes at the end of eighth grade. Predictive models will investigate the ways in which positive and negative reinforcement processes among peers impact future peer affiliations and strength of peer influence. Predictive models will also examine the role of peer harassment and exclusion, and status in the peer network in promoting deviant peer group affiliation, susceptibility to peer influence, and subsequent antisocial behaviors and substance use. For girls, peer exclusion, mood state, and self perceptions are expected to be stronger predictors of changes in peer affiliation and strength of influence than for boys. The protective role of adult monitoring will also be examined. PUBLIC HEALTH RELEVANCE: This research project will develop and evaluate Ecological Momentary Assessment (EMA) methodology for measuring the experiences middle school youth have during non-school times. This methodology will provide researchers with a comprehensive understanding of the ways in which adolescent mood states, perceptions of peer acceptance, peer harassment and exclusion, and status in the peer network impact future affiliation with deviant and prosocial peers and the susceptibility of peer influence. The outcomes will inform prevention efforts by better defining the reinforcement processes among early adolescent peer groups, which will help pinpoint when and how to intervene with at-risk youth to prevent initiation and escalation of substance use.